1. Field of the Invention
This invention relates to the removal of dissolved metals from water, and more particularly concerns polymers and the use thereof for the absorptive removal from aqueous media of trace amounts of dissolved ionized species of metals.
2. Description of the Prior Art
Various techniques are known for removing dissolved ionized metals from water. Activated carbon has long been used for the absorption of dissolved metals from aqueous media. The most significant use of activated carbons for absorption of dissolved metals is in gold mining operations. In such use, activated carbon granules absorb gold in the form of aurocyanide anion produced by the leaching of gold ore with dilute solutions of sodium cyanide. Although effective in said removal of gold from ore, long contact times on the order of 24 hours are required in a tumbling operation.
The removal of nuisance or toxic metals from aqueous streams has generally been accomplished by precipitative methods wherein an ingredient capable of forming an insoluble compound with the dissolved metal is added to the stream. Sufficient interaction time is usually achieved in a holding vessel wherein the insoluble compound forms and grows into a crystalline solid of filterable size. The solid is then removed by settling, filtration or centrifugation, and is usually discarded as a sludge which in fact may require expensive disposal costs.
The separation of dissolved species from water may also be achieved by way of reverse osmosis techniques wherein a pressurized aqueous stream is fed to a selectively permeable membrane. Although reverse osmosis operations are successful in specialized applications, the requisite high pressurization is costly, and the membranes are susceptible to fouling. Also, a concentrated rejectate stream containing the removed species may constitute a significant disposal problem.
Ion exchange resins of strong acid or strong base functionality have been used for the absorption of commonly abundant dissolved cations and anions, respectively. However, commonplace ion exchange resins cannot remove trace ions because they indiscriminately absorb the commonly abundant ions, leaving no residual absorption capacity for trace species. Specialized ion exchange resins, referred to as xe2x80x9cchelationxe2x80x9d resins are known which selectively absorb only xe2x80x9cheavyxe2x80x9d metals. Such specialized resins are usually comprised of beads of styrene/divinylbenzene polymer having grafted iminodiacetic acid groups. xe2x80x9cHeavyxe2x80x9d metals are generally toxic species, usually found in only trace levels in natural waters or industrial effluents. The heavy metals may be further characterized as transition group metals classified in groups I B through VIII B of the Periodic Table, and generally characterized in having incomplete inner rings of electrons or being otherwise capable of existing in more than one valence state.
When a metal-saturated chelation resin must ultimately be disposed of, it is found that the preferred method of disposal, namely incineration, is not a viable option because of the large amounts of volatile aromatic hydrocarbons generated during incineration. The aromatic hydrocarbons are considered to be toxic substances. Although said aromatic hydrocarbons can be removed from the incineration gases by activated carbon, large amounts of the carbon are required.
Chelation resins are not generally capable of removing toxic species to extremely low concentration. For example, where it is desired to remove species such as mercury or lead to concentrations below one part per billion (ppb), chelation resins are usually ineffective. One explanation for said shortcoming is that forces of bonding that hold the metal ion are not sufficiently strong to prevent some dissociation.
The use of aliphatic polymers of nitrilotriacetic acid (NTA) and polyethylenimine (PEI) for the selective absorption of dissolved ions has been disclosed in U.S. Pat. Nos. 3,715,339; 4,332,916; 5,002,984; 5,597,850; and elsewhere. Said PEI-based polymers, having recurring amine groups and iminodiacetic acid groups, absorb cationic species by two different mechanisms. In one mode of function, the amine groups serve as ligands which form a metallo-organic coordination compound with the cation. In a second mode of function, the iminodiacetic acid groups form a at chelation ring which include the absorbed cation. Whereas some cations, such as a copper, cadmium and lead become strongly bound to the polymer, presumably because of bonding via both mechanisms, other heavy metal cations are not sufficiently bound to produce residual concentrations below 1 ppb.
The reaction of PEI with carbon disulfide (CS2) has been disclosed in U.S. Pat. No. 5,387,365 to Moriya et. al. The purpose of the Moriya et. al. modification of PEI is to produce a water-soluble polymer capable of forming a filterable precipitate with dissolved metal cations. Although the Moriya et. al. modified PEI appears to exhibit strong bonding affinity for dissolved metal cations, it does not advance the practicality of metal removal beyond the older technique of merely adding a precipitation agent such as sodium sulfide, followed by filtration.
It is accordingly an object of the present invention to provide a water-insoluble polymer for the rapid and selective absorption of trace amounts of heavy metal species from aqueous solutions.
It is another object of this invention to provide a polymer as in the foregoing object which is amenable to disposal by way of incineration.
It is a further object of the present invention to provide a polymer of the aforesaid nature amenable to disposition in the form of a stationary porous bed through which water can be caused to flow.
These objects and other objects and advantages of the invention will be apparent from the following description.
The above and other beneficial objects and advantages are accomplished in accordance with the present invention by a sulfur-containing water insoluble hydrogel polymer comprised of a reaction produce of NTA and PEI, further reacted with carbon disulfide, said polymer having a physical form capable of producing a stationary porous bed through which water can be caused to flow.